Over the last 50 years amalgam, which for more than a century was dominant in dental filling treatment, has to an increasing extent been replaced by synthetic-based composite materials. The reasons for this change were firstly a desire by patients for aesthetic dental treatment in the side teeth area as well and secondly developments in dental restoration synthetics. In particular synthetic filling materials with 2,2-bis[4-(2-hydroxy-3-methacryloyl-oxypropoxy)phenyl)propane (Bis-GMA) as the base substances have been developed.
Bis-GMA, synthesized from 2 mol glycidyl methacrylate and one mol bipshenol A, contains at both its ends a terminal vinyl group which polymerizes quickly in a radical reaction and results in a cross-linked polymer. The chemical structure of Bis-GMA has a number of special features:                The two aromatic rings of the bisphenol A structure element, which bond with the propane radical are sterically impeded in their rotation. Quantum chemical calculations and experimental research show that the hydrogen atoms at positions 2 and 6 of each aromatic ring overlap with the hydrogen atoms of the two methyl groups of the propane radical, so that even at high temperatures no rotation of the aromatic rings is possible. The molecule thus has the most rigid possible conformation, which results in a comparatively high modulus of elasticity.        The molecule has both free hydroxyl groups and carbonyl oxygen atoms. The presence of these structure elements allows interactions and the development of hydrogen bridge bonds between neighboring Bis-GMA molecules.        
These structural characteristics of the Bis-GMA allow the development of a so-called superstructure based on hydrogen bridge bonds in the polymer, wherein the superstructures because of the rigid monomer conformation are packed extremely densely. This leads to particularly good mechanical characteristics of the material. In addition, the superstructures and the extraordinarily dense packing of the molecular chains ensure that the absorption of water from the oral cavity into the dental material is made considerably harder. Absorption of water leads to a softening of the material and can trigger a hydrolytic decomposition of the ester function of the polymer.
Because of its size and distinctly rigid conformation it is also unlikely that unreacted Bis-GMA, should it leak from the cross-linked polymer, will reach the pulp via the dentinal tubules. Bis-GMA would likewise not be expected to easily pass through biological membranes and bioaccumulate. For these reasons a low toxicity and a generally good biocompatibility is attributed to the Bis-GMA. However, the toxicological characteristics of Bisphenol-A, the starting product for the synthesis of the Bis-GMA, which in traces is always contained in Bis-GMA, have not yet been conclusively clarified.
The molecules of the Bis-GMA, because of their size, are highly space-filling. During the cross-linking therefore the polymerization shrinkage is typically low, for during the radical polymerization of equal substance quantities of Bis-GMA and for example methacrylate fewer bonds are formed than with methyl methacrylate, and thus less bond energy is released.
As a phenomenon, the polymerization shrinkage corresponds to a change in the density occurring during and after cross-linking. This substantially has two causes. On the one hand during polymerization, it is a case of the approximation of the monomeric building blocks of a van der Waals gap to the gap of a covalent bond, and on the other the packing density of the polymer chains is higher than the packing density of the monomers. The shrinkage (in volume) of the reaction resin mass substantially depends on the number of functional groups that have been reacted. The shrinkage takes place both in the fluid state, thus at the very start of the polymerization, as well as during and after gelling. Overall shrinkage comprises a physical and chemical component. While the physical shrinkage is directionally determined and occurs spatially from the outer areas of the polymer, as the temperature drops, internally towards the centre of the molding material, the chemical component is not directionally determined and results solely from the polymer formation. At the start of the polymerization the volume shrinkage can be compensated by the backflow of the material. But within a short time the polymer network has developed so extensively that the gel point is reached, the mobility of the monomers is restricted and backflow of the material is impossible. In this state disadvantageous stresses occur within the material, which when it is used as a dental material lead either to it coming loose from the cavity walls and thus to the formation of marginal gaps or which weaken the material through the formation of volume defects.
In order to counteract the shrinkage, apart from the use of more voluminous monomers such as the Bis-GMA discussed above, the dental composition frequently has volume-stable fillers added. This results in a drop in the number of cross-linkable groups and so the shrinkage and the thermal energy released during bond formation are reduced. Both the use of particularly space-filling monomers and the addition of fillers, however, disadvantageously lead to an increase in the viscosity and thus to a reduction in processability. Therefore conventional dental composite materials contain so-called low molecular reactive thinners, which lower the viscosity thus ensuring the processability of the composite material. Conventional dental composite materials thus consist of a voluminous monomer such as in particular Bis-GMA and low-molecular monomers (reactive thinners) such as for example triethylene glycol dimethacrylate (TEDMA) as well as normal fillers, polymerization initiators and additives.
Through low-molecular monomers (reactive thinners) such as for example triethylene glycol dimethacrylate certain mechanical characteristics of the material are improved, since these molecules even after reaching the gel point are still so mobile that they find reaction partners and through the bonds formed increase the network density of the polymer. TEDMA and other low-molecular monomers of similar structure are as a rule, however, also highly flexible, since these molecules are able to rotate around their ether bonds unhindered. Due to their flexible nature the low-molecular monomers disrupt the homogeneity of the rigid and closely packed Bis-GMA structures, so that apart from some mechanical characteristics being improved others are adversely affected. Therefore to date it has not been possible to indicate Bis-GMA composite materials with minimal polymerization shrinkage and overall good mechanical characteristics, in particular a good abrasion resistance and surface hardness of the cured composite material.
In the past monomer systems have been proposed in which via ring opening reactions the volume contraction is balanced out. Other developments use a cationic ring opening polymerization instead of a radical addition polymerization. Furthermore, liquid crystal monomers, dendritic monomers or organic-inorganic hybrid materials, such as ormocers (organically modified ceramics) have been tested. The conceptual aim is to overcome the volume shrinkage by shifting the balance between broken and newly formed covalent bonds or by differentiation in the packing density between the liquid phase and solid phase.
Also already known is the use of radically polymerizable methacrylic acid or acrylic acid esters with a tricyclo[5.2.1.02,6]-decane (TCD) structure element for the preparation of low shrinkage dental materials. Because of their rigid three-ring conformation and complete steric restriction of mobility, this group of substances is similar to Bis-GMA. In addition, the central aliphatic hydrocarbon element brings about the creation of a considerable hydrophobia, which results in a high water resistance of the polymer. Radically polymerizable methacrylic acid or acrylic acid esters with a tricyclo[5.2.1.02,6]-decane structure element have a very low viscosity and are thus easy to process, and their refractive index also suits the glass ceramic normally used in dental materials.
Dental composite materials containing radically polymerizable methacrylic acid or acrylic acid esters with a tricyclo[5.2.1.02,6]-decane structure element are, inter alia, mentioned in the following printed publications: DE 28 16 823 A1, DE 24 19 887 A1, DE 24 06 557 A1, DE 29 31 926 A1, DE 35 22 005 A1, DE 35 22 006 A1, DE 37 03 080 A1, DE 37 03 130 A1, DE 37 07 908 A1, DE 38 19 777 A1, DE 197 01 599 A1 and DE 699 35 794 T2.
Documents DE 22 00 021 A1, EP 0 023 686 A2, EP 0 049 631 A1, JP 7-206740 A, JP 7-206741 A and JP 11-21370 A likewise disclose radically polymerizable methacrylic acid or acrylic acid esters with a tricyclo[5.2.1.02,6]-decane structure element.
DE 10 2005 021 332 B4 describes composite materials that are claimed to have low shrink force. The composite material disclosed with a total filler content of 80 through 95 wt. % contains                in the filler component                    0.5 through 10 wt. % of non-agglomerated nanofillers with particle sizes of 1 nm through 50 nm,                        and                    at least 60 wt. % of a filler blend of 50 through 90 wt. % of coarse and 10 through 50 wt. % fine particle dental glasses, having a size ratio, with reference to the average particle size of fine particles to coarse particles of 1:4 through 1:30, wherein the proportion of fine particle dental glasses is claimed to be a maximum of 40 wt. % of the filler mixture,                        and as monomer component a mixture of                    60-80 wt. % Bis-GMA or TCD-di-HEMA (bis(methacryloyloxymethyl-)-tricyclo[5.2.1.02,6]decane) or TCD-di-HEA (bis(acryloyloxymethyl)tricyclo[5.2.1.02,6]decane)            10-18 wt. % UDMA (urethane dimethacrylate)            radical TEDMA and/or multifunctional cross-linkers                        up to 1 wt. % initiator(s).        
By the use of non-agglomerated nanofillers and a filler blend of coarse and fine particle dental glasses, through the extensive substitution of TEDMA by UDMA and the optional use of radically polymerizable methacrylic acid or acrylic acid esters with a tricyclo[5.2.1.02,6]-decane structure element and the optional reduction of the initiator quantity according to DE 10 2005 021 332 B4 the achievement of a reduction in the polymerization shrinkage is claimed. This is demonstrated only for TCD-free compositions, however.
In DE 10 2005 053 775 A1 a self-curing or dual-curing, fine-flowing composite material for preparation of a dental liner with polymerization in two stages with two bonding times and delayed polymerization characteristic, which is envisaged for use in the area of the cavity wall in a thin coating, is disclosed. Preferred monomer components include TCD-di-HEMA and TCD-di-HEA.
EP 1 935 393 A2 and DE 10 2006 060 983 A1 relate to dental composites comprising radically polymerizable acrylic acid esters with a tricyclo[5.2.1.02,6]-decane structure element. The higher degree of polymerization of these is claimed to be advantageous for the mechanical characteristics of the composites. However, acrylate monomers are considered unsuitable for dental applications because of their harmful toxicological characteristics. Following curing the cross-linked TCD acrylate monomers are nevertheless claimed to have a very favorable biocompatibility. This document also describes compositions which substantially are Bis-GMA-free.
EP 2 016 931 A2 and DE 10 2007 034 457 A1 also described dental composites comprising radically polymerizable methacrylic acid or acrylic acid esters with a tricyclo[5.2.1.02,6]-decane structure element, wherein the monomer components must contain both Bis-GMA and TCD-di-HEMA or TCD-di-HEA.
WO 03/035013 A1 and DE 602 16 951 T2 relate to dental adhesive compositions for binding of dental restoration means to dentin and/or tooth enamel. In these documents inter alia the preparation of 3,(4),8,(9)-bis(2-propanamidomethyl)tricyclo[5.2.1.0]2,6-decane is described.